Batteries comprising a positive electrode and a negative electrode separated by an electrolyte comprising a lithium salt in solution in a solvent are widely known. The operation of these batteries is provided by the reversible circulation of lithium ions in the electrolyte between the electrodes. The positive electrode is generally composed of a composite material comprising an active material, a binder, a material conferring electron conduction and optionally a compound conferring ionic conduction. The compound conferring electron conduction can be a carbon black which does not catalyze the oxidation of the electrolyte at a high potential.
The use is known, in particular from FR-2 831 715, of a lithium vanadium oxide Li1+αV3O8 (0.1≦α≦0.25) as positive electrode active material. The use is also known of a β-LixV2O5 compound as positive electrode active material in lithium batteries. However, their use in a composite electrode requires the addition of a material conferring electron conduction, for example carbon.
Various attempts have been made to introduce carbon into a positive electrode active material during the preparation of said active material, in order to improve the contacts between the active material and the carbon and consequently the electron conductivity of the composite electrode.
Takashi Watanabe et al. [Solid State Ionics, 151, 1-4, (2002)] describe the preparation of a V2O5.nH2O-carbon nanocomposite by a process comprising the stages of: preparation of a V2O5.nH2O sol by reaction of aqueous hydrogen peroxide solution with vanadium metal; addition of water and of acetone to the V2O5.nH2O sol in order to stabilize it; addition of carbon; drying the mixture obtained in order to form an electrode therefrom. The material obtained is composed of carbon grains coated with V2O5.nH2O. However, the good electrochemical performances of the material are obtained only with a very high C/active material ratio by weight, of the order of 1.4, which is extremely unfavorable to any industrial application.
Huan Huang et al. [Angew. Chem. Int. Ed., 2001, 40, No. 20] describe the preparation of a V2O5.nH2O-carbon nanocomposite by a process comprising the following stages: preparation of vanadic acid by passing a solution of sodium metavanadate NaVO3 through an H+/Na+ ion-exchange column; production of the V2O5.nH2O xerogel by polycondensation of the vanadic acid; acid treatment of the carbon in order to functionalize the surface; optionally grafting of polyethylene glycol to the pretreated carbon; drying the gel at ambient temperature; dissolution of the xerogel in order to obtain a sol with the help of ultrasound to accelerate the process; mixing the treated carbon and the sol with magnetic stirring until the solvent has completely evaporated. This process requires approximately two days and the tedious and expensive use of an ion-exchange resin. The material obtained is composed of carbon particles coated with V2O5.nH2O. During the preparation of a positive electrode from the nanocomposite, additional carbon is added in a proportion of 10% by weight.
EP-104 918 describes the preparation of a LiFePO4-graphitized carbon nanocomposite in which a thin graphite film coats the surface of the active material. The nanocomposite can be obtained by a 3-stage process: synthesis of LiFePO4; coating of the LiFePO4 grains with a carbonaceous compound which is the precursor of the graphitized carbon; pyrolysis of the coated grains at 700° C. under argon to convert the polymer to conducting graphitized carbon. The nanocomposite can also be obtained in a single stage consisting in carrying out a pyrolysis under argon of a mixture of the LiFePO4 reactants and of the precursor carbonaceous compound. The results show a marked improvement in terms of capacity and of power due solely to a greater effectiveness of the carbon-active material contact. In all cases, 10% by weight of additional carbon is added during the preparation of a composite electrode.
Huang H et al., [Electrochem. Solid State Lett., 4 (10), A170-172 (2001)] and [Adv. Mater., 2002, 14, No. 21, November 4] respectively describe the preparation of LiFePO4-graphitized carbon and LiV2(PO4)3-graphitized carbon nanocomposites by a two-stage process. The precursors of the active material are mixed with a carbon gel which results from the polymerization of resorcinol-formaldehyde. A heat treatment at 700° C. under nitrogen for 5 h is necessary to obtain the nanocomposite. Their process results in the grains of the active material being coated with carbon. The use of resorcinol makes the process expensive. In addition, during the preparation of the composite electrode, it is necessary to add additional conducting carbon.